1. Field of the Application
The present invention relates to pipe production processes and equipment and, more particularly, to a method and mill for reduction and sizing of welded pipes.
The invention is applicable to the reduction and sizing of pipes of continuous Fretz-Moon mills and electric-weld pipe mills.
Being applied to mills with two-high mill stands both in operation and under construction, the invention accounts for a greater strength of the weld and a more accurate pipe wall size, i.e. a more even pipe wall gauge.
2. Description of the Prior Art
Today, welded pipes are manufactured on a large scale in most industrially developed countries.
Continuous welding of pipes in combination with subsequent reduction and sizing is more productive and less expensive than the manufacture of seamless pipes. However, the mechanical and physical properties of the weld and areas adjacent to the weld are inferior to those of the base metal.
A major problem in this field is to find ways and means to improve the strength of the weld and bring the properties of welded pipes as close as possible to those of seamless pipes, while increasing the productivity and reducing the production costs.
At present, the commonest pipe production process comprises continuous welding with subsequent reduction of the hot pipe and is referred to as the Fretz-Moon pipe process.
According to this method, a band is heated to a temperature sufficiently high for a welding operation and formed in a first stand into a tubular blank, the forming angle being 220.degree. to 270.degree.. In a second stand, the blank is welded into a pipe which is reduced and sized in other stands.
The two-roll oval passes of the stands alternate so that the next pass is turned through 90.degree. in relation to the previous pass.
The method under review is disadvantageous in that it causes a great non-uniformity of deformation in the pass tapers and cyclic bends of portions of a pipe passing in the plane of apices and tapers of the passes; this is due to the fact that the angle of convergence of the cross-sectional portions of the pipe varies from 180.degree. at the apex of the pass to 160.degree. at the pass taper.
This results in sharp bends of the pipe walls, which in turn, account for great tensile stresses of the outer layers of the pipe in the pass taper and of the inner layer of the pipe as the portion of the pipe passes on to the apex of the next pass.
As a weld is being formed, its strength is quite low. This is due to a high temperature of the edges being welded (which may be as high as 1,480.degree. C), a low welding pressure and an increase of impurities in the weld zone. Under such conditions, cyclic bends of the portion of the pipe with the weld, which occur before the weld cools, reduce the weld strength and account for incomplete fusion due to the appearance of hot cracks on the outer and inner surfaces of the pipe.
The great non-uniformity of deformation in the tapers the oval passes accounts for an increased thickness, as well as an uneven cross-sectional gauge.
Another wide-spread pipe production process is the welding of pipes from cold band. Prior to the welding operation, the edges are heated by direct current, high-frequency current, etc. The welding is followed by cold or hot reduction.
This method, too, suffers from all the above disadvantages. Besides, the cold reduction hardens the pipe portions subjected to cyclic bending, which reduces the maximum permissible degree of drawing and reduces the efficiency of the pipe production process.
There is known a method for deformation of pipes in rolls whose passes are formed by conjugate arcs of three radii. This method is used to manufacture pipes of a rectangular section. As the pipe passes from pass to pass, two radii increase to infinity, whereby the planes of the tube rectangular in cross section are produced. The conjugate radius is reduced to the radius of the edge round-off of this profile.
However, the above method is not applicable to the reduction and sizing of round-section pipes.
There is known a method for reducing and sizing pipes in rolls with double-radius passes. This method is disadvantageous in that a pipe cannot be deformed so that its radius at the pass taper should be equal to the radius at the apex of the foregoing pass.